1. Field of the Invention
The present invention relates to voltage nonlinear resistors and methods for fabricating the same, and to varistors.
2. Description of the Related Art
As the sizes of circuits are reduced and reference frequencies are increased, there are demands for electronic components which are small and suitable for higher frequencies. As the driving voltages for circuits are decreased, there are also demands for electronic components which can cope with decreased voltage. This trend also applies to varistors as abnormal-voltage absorbing devices.
As voltage nonlinear resistors, SiC-based varistors, ZnO-based varistors and SrTiO3-based varistors are generally known. With respect to ZnO-based varistors and SrTiO3-based varistors, monolithic chip varistors with a driving voltage of 3.5 V or more have been developed and commercially available.
In order to make a varistor suitable for higher frequencies and to use the varistor as a noise-absorbing device in a signal circuit, etc., the capacitance of the varistor must be decreased. In order to make the varistor suitable for decreased voltage, the varistor voltage must be reduced.
However, the conventional ZnO-based varistor has an apparent relative dielectric constant of 200 or more, and the apparent relative dielectric constant of the SrTiO3-based varistor is higher than that of the ZnO-based varistor, at several thousands to several ten thousands. Therefore, in order to decrease the capacitance of the varistor, the total area of electrodes must be greatly decreased or the number of particle boundaries must be increased by increasing the thickness of the device between the electrodes. However, if the total area of electrodes is decreased, the surge current capacity is also decreased, and if the thickness of the device between the electrodes is increased, the varistor voltage is increased. If the varistor voltage is decreased, the capacitance of the varistor is further increased, and therefore, it has been difficult to make the low voltage and the low capacitance requirements compatible with each other.
With respect to the SiC-based varistor, since the apparent relative dielectric constant is low, the capacitance can be easily decreased. However, the SiC-based varistor has a lower voltage nonlinear coefficient a in comparison with other varistors. For example, in the ZnO-based varistor or the SrTiO3-based varistor, the voltage nonlinear coefficient is several tens, while the SiC-based varistor has a voltage nonlinear coefficient of 8 at most. For the reasons described above, a voltage nonlinear resistor in which the capacitance is decreased, the voltage nonlinear coefficient a is increased and the varistor voltage is decreased, is not available at present.
Accordingly, it is an object of the present invention to provide a voltage nonlinear resistor in which the capacitance is decreased, the voltage nonlinear coefficient a is increased and the varistor voltage is decreased.
In order to achieve the object described above, the present inventors have conducted various experiments and examinations with respect to voltage nonlinear resistors composed of aggregates of n-type semiconductive SiC particles doped with impurities, such as N2. As a result, it has been found that electrical characteristics of the voltage nonlinear resistors depend on the surface state of the SiC particles, and that oxygen must be diffused into the surfaces of SiC particles to a depth of about 100 nm or less and at least one element selected from the group consisting of A1 and B must be diffused into the surfaces of SiC particles to a depth of about 5 to 100 nm.
In one aspect of the present invention, a voltage nonlinear resistor is composed of an aggregate of silicon carbide particles doped with impurities, in which oxygen and at least one of aluminum and boron are diffused in the vicinity of the surfaces of the silicon carbide particles, the diffusion length of the oxygen is about 100 nm or less from the surfaces of the silicon carbide particles, and the diffusion length of at least one of the aluminum and the boron is in the range of about 5 to 100 nm from the surfaces of the silicon carbide particles.
Preferably, the diffusion length of the oxygen is in the range of about 25 to 85 nm from the surfaces of the silicon carbide particles. Preferably, the diffusion length of at least one of the aluminum and the boron is in the range of about 25 to 70 nm from the surfaces of the silicon carbide particles.
Preferably, the element ratio of silicon being present within about 10 nm from the surfaces of the silicon carbide particles to the at least one of the aluminum and the boron is about 1:0.5 to 3. By modifying the surfaces of the silicon carbide particles to such a state, it is possible to obtain a superior voltage nonlinear resistor which has a small capacitance and high xcex1, and which is resistant to surge and static electricity.
The average particle size of the silicon carbide particles is preferably in the range of about 0.3 to 70 xcexcm and more preferably in the range of about 1 to 30 xcexcm. By setting the average particle size of the silicon carbide particles in such a range, the varistor voltage can be controlled.
In another aspect of the present invention, a method for fabricating a voltage nonlinear resistor includes the steps of: adding at least one of aluminum and boron to silicon carbide powder doped with impurities; and heat-treating mixed powder obtained in an oxidizing atmosphere in order to form silicon carbide particles based on the silicon carbide powder, to diffuse at least one of the aluminum and the boron into the surfaces of the silicon carbide particles and to oxidize the surfaces of the silicon carbide particles. In such a case, the heat-treating temperature is preferably set at about 1,100 to 1,500xc2x0 C.
In another aspect of the present invention, a method for fabricating a voltage nonlinear resistor includes the steps of: adding at least one of aluminum and boron to silicon carbide powder doped with impurities; heat-treating mixed powder obtained in a non-oxidizing atmosphere in order to form silicon carbide particles based on the silicon carbide powder and to diffuse at least one of the aluminum and the boron into the surfaces of the silicon carbide particles; and oxidizing the surfaces of the silicon carbide particles formed by the heat treatment. In such a case, the heat-treating temperature is preferably set at about 800 to 1,500xc2x0 C.